A new ATPase assay was developed to determine anzymatic activity in myc antigen immune complexes. ATPase activity was demonstrated using gag and myc antisera against myc proteins solubilized from the nuclei and nuclear matrix of Q8 cells, a source of p110. The enzyme required a neutral pH, Mg, detergent, dithiothreitol, and KC1. The enzyme cleaved only the terminal phosphate from ATP. ATP hydrolysis could be cmpetitively inhibited by ATP, ADP, dATP and dADP. The immune complex also expressed GTPase activity. Nuclear extracts from Q8, QMH2 and OK10 cells precipitated with gag antiserum produced about the same specific ATPase activity. ATPase activity co-migrated with p110 from Q8 nuclei on a molecular sizing column.